Baking fluid sim to shape keys
It is not possible to convert a fluid simulation to shape keys. The fluid sim is in essence creating a new mesh every frame. Even though it looks like it is a mesh changing a tiny bit from frame to frame, it is not. Shape keys blend between two meshes that have the same vertex count, also they need to have the same vertex order, but they will technically still work even if the order is messed up. Since the vertex count from frame to frame changes you cannot convert a baked fluid simulation to shape keys.

Also you should simulate the two together as a single simulation because the results of one sim being simulated with interaction to another after the one is already simulated is not going to look very realistic anyway.